Light-weight ceramic insulation

ABSTRACT

Ultra-high temperature, light-weight, ceramic insulation such as ceramic tile is obtained by pyrolyzing a siloxane gel derived from the reaction of at least one organo dialkoxy silane and at least one tetralkoxy silane in an acid or base liquid medium. The reaction mixture of the tetra- and dialkoxy silanes may contain also an effective amount of a mono- or trialkoxy silane to obtain the siloxane gel. The siloxane gel is dried at ambient pressures to form a siloxane ceramic precursor without significant shrinkage. The siloxane ceramic precursor is subsequently pyrolyzed, in an inert atmosphere, to form the black ceramic insulation comprising atoms of silicon, carbon and oxygen. The ceramic insulation, can be characterized as a porous, uniform ceramic tile resistant to oxidation at temperatures ranging as high as 1700° C. and is particularly useful as lightweight tiles for spacecraft and other high-temperature insulation applications.

ORIGIN OF INVENTION

The invention described herein was made in the performance of work undera NASA Contracts NAS2-97028 and NAS2-98067 and is subject to Public Law96-517 (35 U.S.C. §200 et seq.). The contractor has elected to retaintitle in the invention.

BACKGROUND OF INVENTION

1. Field of the Invention

This invention relates to an ultra high-temperature, lightweight ceramicinsulation such as porous ceramic tile containing silicon, carbon andoxygen. More particularly, the invention relates to a lightweight,ceramic insulation containing silicon, carbon and oxygen capable ofretaining its shape, strength and physical properties when exposed to anoxidizing environment at temperatures as high as 1700° C. Morespecifically, this invention relates to the sol-gel process of preparingceramic insulation which comprises an insulation product derived fromthe reaction of multifunctional silanes to form a small-pore, wetsiloxane gel followed by drying the gel at ambient pressures withlimited shrinkage and, subsequently heating or pyrolyzing the dried gel,in an inert atmosphere, to form the high-temperature, lightweightceramic insulation.

An important use for the insulation of this invention includes, forexample, the space vehicles, such as the space shuttle which leaves andreenter the earth's atmosphere and therefore requires exterior thermalinsulation. The operation of the space shuttle requires the developmentof lightweight and thermally efficient exterior insulation capable ofwithstanding a variety of environments. During reentry into the earth'satmosphere, the insulation must maintain the vehicle's exteriorstructure below 175° C. while experiencing substantial aeroconvectivethermal environments which heat the surface of the insulation totemperatures in excess of 1,000° C. In space, the thermal protectionmust insulate the vehicle from the cold (e.g., −70° C.) experiencedwhile in orbit. In addition to thermal and aeroconvective environments,the insulation must be able to withstand the mechanical stressassociated with launch vibrations, acoustics, structural movement of thesurface of the vehicle, and the landing impact.

The use of state-of-the art thermal insulation, lightweight ceramictiles, developed by Lockheed (LI-900), and NASA/Ames Research Center(AETB, AIM, FRCI, etc.), are all limited to temperatures of use at about1300° C. in an oxidizing environment. For applications which experiencetemperatures above 1300° C., a dense ceramic insulation must be usedwhich adds a substantial weight penalty. Presently, the thermalinsulation used for protecting space vehicles includes both the rigidand flexible ceramic insulation with a carbon composite being used onthe leading edges of the vehicle. However, these ceramic carboncomposites must be very porous in order to maintain the weight at areasonable low level. This can be accomplished by using theultrahigh-temperature, stable, lightweight ceramic insulation of thisinvention.

2. Description of the Prior Art

In general, low-density insulations are required to thermally protectthe structure of the Space Shuttle from the high temperatures normallyencountered during atmospheric entry. The material developed for theShuttle was, a rigidized fibrous insulation, called reusable surfaceinsulation (RSI). Its density and conductivity were optimized (minimumconductivity and weight) to keep the thermal protection system weight aslow as possible, consistent with adequate mechanical properties toincrease the resultant payload capability of the vehicle. Acharacteristic of a successful insulation is the high-thermal shockresistance, required to survive the rapid temperature changes and highthermal gradients normally incurred during entry. The temperaturelimitation of the prior materials and the desirability to improve themechanical properties of these materials are the reasons to developalternate materials. There is a need to develop alternate insulationsystems for advanced earth-entry vehicles. These needs are relative tothe state-of-the-art materials and include improved mechanicalproperties, higher-temperature capability, equivalent thermal shockresistance, low-thermal conductivity, and adequate morphologicalstability. Presently, composite insulating materials intended for use onorbital reentry vehicles, such as the Space Shuttle, consist of acoating in combination with low density insulation substrates. Examplesof these composites and their use are provided in U.S. Pat. No.4,148,962, issued Apr. 10, 1979; U.S. Pat. No. 3,955,034, issued May 4,1976, and U.S. Pat. No, 4,612,240, issued Sept. 16, 1986.

More specifically, details regarding prior ceramic insulations aredisclosed in various other U.S. patents. For example, U.S. Pat. No.5,618,766 discloses lightweight ceramic compositions comprising a porouscarbon preform. The carbon preform contains a tetralkoxy silane, adialkoxy silane and a trialkyl borate. Pat. No. 4,713,275 relates to aceramic tile for use in a thermal protection system, employing a ceramiccloth having additional ceramic material deposited therein. Pat. No.4,804,571 relates to a thermal protection system for reentry vehicles orhigh speed aircraft including multiple refractory tiles of varyingthickness defined by thermal requirements at the point of installation.Pat. No. 4,100,322 relates to a high thermal capacity fiber-resin-carboncomposite having a polymer resin filler. The composite is prepared byimpregnating a woven fabric of carbon or graphite yarn with a resin,curing the resin, pyrolyzing the impregnated fabric and re-impregnatingthe resulting fiber-porous carbon char composite with resin. Pat. No.4,605,594 relates to a ceramic article including a woven ceramic clothhaving a non-porous core and a porous periphery prepared by treatingwith an acid. The porous periphery can be infiltrated with materialssuch as metal, a metal oxide, a catalyst and an elastomer. The articlescan be used as fiber optic elements, catalyst supports and heatresistant tiles for aerospace purposes. Pat. No. 3,533,813 relates to alow density, nonstructural ceramic employing a porous ceramic support incombination with organic fillers. The process includes burning off theorganics to form pores within the ceramic. This process reduces the massof the composite, thereby reducing its density while maintaininginherent strength.

Further, it is generally known in the art that a gel consists of atenuous solid skeleton immersed in liquid. When the liquid issupercritically extracted in an autoclave, the skeleton is called anaerogel, and the volume formerly occupied by liquid becomes porosity. Ifthe wet gels are dried at ambient pressure, the gels shrink to a smalldry gel with higher density. Silica aerogels are porous, and are verygood insulation materials, prepared from wet gels (alcogels) by thesupercritical drying method. U.S. Pat. No. 4,327,065 describes themethod of preparing silica aerogel. The preparation is effected by meansof hydrolysis of a tetraalkoxysilane in an alcohol, in the presence of acatalyst for the formation of an alcogel which is aged and washed withalcohol to remove water. The alcogel is thereafter treated in anautoclave by means of a temperature increase to above the critical pointof the alcohol, isothermic pressure drop by means of the release ofalcohol vapor, and then a temperature drop. U.S. Pat. No. 4,610,863describes an improved supercritical drying process for forming silicalaerogel. The improvement includes the additional step, after alcogelsare formed, of substituting a solvent, such as CO₂, for the alcohol inthe alcogels, the solvent having a critical temperature less than thecritical temperature of the alcohol. The resulting gels are dried at asupercritical temperature for the selected solvent, such as CO₂.

SUMMARY OF THE INVENTION

In comparison to the teaching's of the prior art, this invention relatesto lightweight, high-temperature ceramic insulation comprising atoms ofsilicon, carbon and oxygen derived from the reaction of at least oneorganodialkoxy silane and at least one tetraalkoxy silane to form asiloxane gel in the presence of a catalyst in a liquid medium such asalcohol. In addition, the reaction mixture can include effective amountsof at least one alkyl trialkoxy silane and/or a monoalkoxy silane. Moreparticularly, the invention relates to an oxidation resistant, ceramicinsulation material containing silicon, carbon and oxygen and to themethod of preparing a ceramic insulation e.g. ceramic tile capable ofretaining its shape and strength when exposed to an oxidizingenvironment at temperatures ranging up to about 1700° C. The methodresults in a unique product derived from the reaction of at least onedialkoxy silane and at least one tetraalkoxy silanes such as the di- andtetra-functional silanes to form wet alcogels, in the presence of acatalyst in a liquid medium followed by drying the alcogel at ambientpressures and subsequently heating or pyrolyzing the dried gel, in aninert atmosphere, to temperatures ranging from about 900° C. to 1500° C.to form the ceramic insulation.

The preferred di- and tetra-functional alkoxides are the siliconalkoxides having di- and tetra-oxygen functionality wherein the siliconalkoxides have two and four Si—O bonds, respectively. The silanesparticularly useful in the practice of this invention include acombination of silanes with tetra- and di- oxygen functionality of thegeneral formula Si(R¹O)₄ and (R²O)₂−Si—R⁴R³ wherein R¹ and R² are thesame or different and represent saturated or unsaturated alkyl groups orradicals of 1-12 carbons and wherein R⁴ and R³ are different or the sameas R¹ and R². Preferably the radicals R⁴ and R³ are the same ordifferent radicals of 1 to 8 carbons e.g. 1-6 carbons and include thealkyl, alkenyl, aryl, alkaryl, and aralkyl radicals. One of the R⁴ or R³can be hydrogen. The radicals can be hydrocarbon groups i.e. (—CH) ofcarbon and hydrogen and include the straight or branched chains, and thesaturated or unsaturated radicals of 1 to 12 carbon. In general, thenumber of carbon atoms in the hydrocarbon groups range from 1-8 and morepreferably from 1-4 carbons.

Gelation of the mono-, tri-, di- and tetraalkoxy silanes is catalyzed bythe addition of catalytic amounts of a water soluble acid or base, suchas a mineral acid i.e. H₂NO₃, H₂SO₄, HCl, or a base such as KOH, NaOH,NH₃, ammonium hydroxide or a water soluble amine and various other knownwater soluble acids or bases. In addition to water or aliphatic alcoholsand mixtures thereof, in some applications it may be appropriate to useadditional solvents that are either water-soluble or dispersible as theliquid reaction media such as acetone or any other such solvent. Thesesolvents would expedite fast drying and serve to dilute or furtherliquefy the siloxane gel to control the density of the dry gel.

In the instant process, the siloxane gel is dried prior to pyrolysis atambient pressures without using an expensive autoclave. The dried gelwill not shrink, significantly, if the reaction time and the ratio ofthe reagents are controlled. A ceramic tile is formed by pryolyzing orheating the dried sponge gel, in an inert atmosphere i.e. an inert gasat temperatures ranging up to about 1500° C. e.g. preferably from 900°C. to 1300° C. When a high pyrolyzing temperature is used, e.g. rangingup to 1500° C., the time required for pyrolyzation is substantiallydecreased. However, where there is need for lower production costs, thelower temperatures can be used for longer periods. This may beparticularly important for a very large substrate because very largebaking ovens tend to have a lower maximum temperature. Moreover, whenthe dried gel is relatively thick, the time required for uniformpyrolyzation throughout the entire monolith will require more extendedperiods of heating. This will be easily ascertained by one of ordinaryskill in the art. A large monolith may require several days of treatmentfor the center regions of the gel to be fully pyrolyzed. The molar ratioof the silicon, oxygen and carbon atoms in the reaction product(siloxane) is determined by the molar ratio of the di- and teraalkoxysilanes in the reaction and in some instances by the molar ratio of thetetralkoxy silane and the carbon content of the Si—C bonds in thedialkoxy silanes.

The lightweight, oxidation resistant, ceramic insulation or tiles ofthis invention, useful in advance space vehicles, are made by formingthe gel followed by drying and subsequent pyrolysis. Specifically, thisinvention is directed to a process of preparing an oxidation-stable,high temperature, porous ceramic insulation e.g. tiles which comprisesreacting, in a liquid medium, effective amounts of (a) at least onetetraalkoxy silane having the formula:

Si(OR¹)₄

wherein R¹ is a saturated or unsaturated organic radical having 1 to 12carbons with (b) effective amounts of at least one dialkoxy silanehaving the formula:

(R²O)₂—Si—R⁴R³

wherein R² is a saturated or unsaturated organic radical having 1 to 12carbons and R⁴ and R³ are saturated or unsaturated, either the same ordifferent organic radicals having 1 to 12 carbons to obtain a siloxanegel drying said siloxane gel and ( c) subsequently pyrolyzing thesiloxane gel in an inert atmosphere at temperatures ranging up to about1500° C. to produce the porous ceramic insulation. The reaction ratio ofthe tetraalkoxy silane to the dialkoxy silane ranges from about 1.0 partby weight of the tetraalkoxy silane to about 0.1 to 2.0 and preferably0.7 to 1.5 parts by weight of the dialkoxy silane.

The reaction between the tetraalkoxy silane and the dialkoxy silane maycontain at least one alkyl trialkoxy silane having the formula:

R⁵—Si(OR⁶)₃

wherein R⁵ is a saturated or unsaturated alkyl radical of 1 to 12carbons and R⁶ is either the same or a different alkyl radical having 1to 12 carbons. The reaction between the tetraalkoxy silane and thedialkoxy silane may also contain at least one alkyl monoalkoxy silanehaving the formula:

R⁷ ₃—Si—OR⁸

wherein R⁷ is a saturated or unsaturated alkyl radical of 1 to 12carbons and R⁸ is either the same or different alkyl radical of 1 to 12carbons. Where the R⁴ and R³ alkyl radicals of the dialkoxy silane have1-4 carbons, the ratio in the reaction mixture is about 1.0 part byweight of the tetraalkoxy silane to about 0.1 to 2.0 part by weight ofthe dialkoxy silane to about 0.01 to 0.5 parts by weight of thetrialkoxy silane. Where the R⁷ and R⁸ are alkyl radicals of 1-4 carbons,the ratio in the reaction mixture is about 1.0 part by weight of thetetraalkoxy silane to about 0.1 to 2.0 parts by weight of the dialkoxysilane to about 0.01 to 0.5 parts by weight of the alkyl monoalkoxysilane.

The oxidation-stable, high temperature, porous ceramic insulationmaterials of this invention can be characterized as being derived frompyrolyzed siloxane gels consisting essentially of silicon, carbon, andoxygen in the mole ratios of about 1.0 mole of silicone, to about 0.1 to1.0 mole of carbon to about 0.8-2.0 moles of oxygen. This ceramicinsulation is further characterized as having black particle sizesranging from about 1.0 to 50 um., void sizes ranging from about 10 to100 um. and is capable of retaining its insulation and other physicalproperties at temperatures ranging up to about 1700° C.

Accordingly, it is an object of this invention to provide hightemperature, lightweight ceramic insulation for use at temperatures ashigh as 1700° C.

It is another object of this invention to provide a method of preparinghigh temperature, lightweight ceramic insulation derived from thereaction of alkoxy silanes containing silicon, oxygen and carbon for useon nuclear reactors, spacecraft nose tips, and various other leadingedges.

It is another object of this invention to provide ceramic insulationtile having high temperature characteristics, lightweight, high tensilestrength and capable of being formed into any desired shape.

It is a further object of this invention to provide ceramic insulationtile comprising siloxanes derived from the reaction of alkoxy silanesincluding the mono-, di-, tri-, and tetraalkoxy silanes, and preferablyfrom the reaction of dialkoxy silanes and tetralkoxy silanes, for use onspace vehicles and other insulation applications, having improved hightemperature characteristics, lightweight and high tensile strength.

These and other objects of this invention will become apparent from afurther and more detailed description of the invention as follows.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to a black oxidation resistant, lightweightamorphous high-temperature ceramic insulation comprising silicon, carbonand oxygen atoms and to the method for preparing said insulation whichcomprises the formation of a siloxane gel derived from the reaction ofdi- and tetraalkoxy silanes in a liquid medium such as water and/oralcohol to form a siloxane gel, followed by drying the gel at ambientpressure without using an autoclave, and subsequently heating orpyrolyzing the dried sponge gel, in an inert atmosphere, to form theblack ceramic insulation. In one embodiment, the siloxane gel in variousamounts of alcohol and water, is dried at ambient pressure to form aceramic siloxane precursor. The ceramic siloxane precursor issubsequently pyrolyzed, in an inert atmosphere i.e. inert gas, to formthe black ceramic insulation of this invention. The insulation ischaracterized as having limited shrinkage in size, shape andconfiguration and is stable at ultra-high temperatures. Various amountsof solvent i.e. water, alcohol and mixture thereof in any ratio can beused to achieve the desired density and strength of the high-temperatureoxidation stable ceramic insulation.

For purposes of this invention, the preferred di- and tetrafunctionalalkoxide reactants include the alkoxides of silicon having two and fourSi—O bonds, respectively. Particularly preferred silanes comprise acombination of silanes with tetra- and dioxygen functionality having thegeneral formula Si(OR¹)₄ and (R²O)₂—Si—R⁴R³ wherein R¹, R², R³ and R⁴are the same or different and represent organic radicals of 1-12carbons, such as an alkyl, alkenyl, aryl or substituted radical of 1 to12 carbons such as an alkyl, aryl or substituted radical of 1 to 8carbons. The term hydrocarbons comprise carbon and hydrogen (—CH) whichmay be straight or branched chain, saturated or unsaturated radicals. Ingeneral, the number of carbon atoms in the hydrocarbon or organo groupsrange from 1-12 and preferably from 1-8 and more preferably 1-4 carbonse.g. 1-2 carbons. The R¹, R², R³, and R⁴ groups of the above formulaeare preferably lower alkyl groups, e.g. 1 to 8 carbons such as methyl,ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, hexyl, heptyl, etc.,the isomers, and mixtures thereof and include the alkenyl or vinylunsaturated groups such as vinyl, divinyl, propenes, butenes, etc. andvarious mixture thereof in various ratios.

Specific examples of some of the preferred silanes useful for preparingthe siloxane gels of this invention include the tetraalkoxy silanes suchas tetramethoxy silane or tetraethoxy silane. Some of the preferreddialkoxy silanes include the diethyldiethoxy silane (C₂H₅)₂Si(OC₂H₅)₂,diethydibutoxysilane (C₂H₅)₂Si(OC₄H₉)₂, dimethyldiethoxy silane(CH₃)₂Si(OC₂H₅)₂, dimethyldimethoxy silane (CH₃)₂Si(OCH₃)₂,diphenyldimethoxy silane (C₆H₅)₂Si(OCH₃)₂, vinylmethyldiethoxy silane(CH2:CH)(CH₃)Si(OC₂H₅)₂ and various combination thereof in any ratios.

Tthe siloxane sol-gels of this invention are prepared by reacting thealkoxy silanes in the presence of an aqueous catalyst (up to about 10%e.g. 0.1 to 2.0% by weight of the liquid medium) in an aqueous-alcoholor alcohol medium The weight ratio between the dialkoxy silane and thetetraalkoxy silane ranges from about 1.0 part by weight of thetetraalkoxy to 0.1 to 2.0 part by weight of the dialkoxy silane, andpreferably from about 0.7 to 1.5 parts by weight. The alcohol mediainsures that a homogeneous sol-gel is obtained. While it is convenientto use the lower alcohols such as ethanol, other lower aliphaticalcohols of 1 to 8 carbons e.g. 1 to 4 carbons may be used alone or inadmixture in any ratio. Examples of the preferred alcohol media includemethanol, ethanol, propanol, isopropanol, butanol, sec- and isobutanol,pentanol, and any mixture of the lower alcohols alone or with water inany ratio. Although the siloxane sol may be gelled by aging at ambienttemperatures or by heating, it is preferred to catalyze gelation by theaddition of a catalytic amount e.g. 0.1 to 2.0% by weight of a dilutemineral acid e.g. HNO₃, HCl, etc. or a base such as NaOH, KOH, ammoniumhydroxide or a low molecular weight amine etc. to the reaction mixture.Mineral acids such as nitric acid or a base such as ammonium hydroxideare particularly useful as gelling agents. Gelation will occur atambient conditions, but heating to temperatures of from about 40°-90° C.is preferred also in addition to the use of the acid or base catalyst.After gelation, the gel is dried in an oven or at ambient conditions toform the siloxane precursor. Vacuum drying (e.g., overnight at 70-100°C.) is not necessary to insure that the gel does not shrink,significantly, prior to pyrolyzing the gel in an inert atmosphere.

In addition, the reaction mixture of this invention may contain smallbut effective amounts of at least one mono- or trialkoxy silane incombination with the di- and tetraalkoxy silanes, in the ratios of about1.0 part by weight of the tetraalkoxy silane to about 0.1 to 2.0 part byweight of the dialkoxy silane to about 0.01 to 1.0 part by weight of thetrialkoxy silane or to about 0.01 to 1.0 part by weight of themonoalkoxy silane. The tri- and monoalkoxy silanes used in combinationwith the tetra- and dioxygen functionality silanes have the generalformula R⁵—Si—(OR⁶)₃ and R⁷ ₃—Si—OR⁸ wherein R⁵, R⁶, R⁷ and R⁸ are thesame or different and represent organic radicals of 1-12 carbons. Thenumber of carbon atoms in the hydrocarbon or organo groups range from1-12 and preferably from 1-8 and more preferably 1-4 carbons or 1-2carbons. The R⁵, R⁶, R⁷, R⁸, organic groups of the above formulae arepreferably lower alkyl radicals e.g. 1 to 8 carbons such as methyl,ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, hexyl, heptyl oroctyl, the isomers, and mixtures thereof including the alkenyl or vinylunsaturated groups such as vinyl, divinyl, propenes, butenes, etc. andvarious mixture thereof. Some specific examples of the mono- andtrialkoxy silanes useful for preparing the siloxane gels of thisinvention include the alkyltrialkoxy silanes such as methyl trimethoxysilane CH₃Si(OCH₃)₃, ethyltrimethoxy silane C₂H₅Si(OCH₃)₃,methyltriethoxy silane CH₃Si(OC₂H₅)₃ and various monalkoxy silanes suchas trialkylethoxy silanes and trialkylmethoxy silanes etc.

The ceramic insulation is formed by heating the dried sponge whitesiloxane gel at temperatures ranging up to about 1500° C. in an inertatmosphere. The inert atmosphere includes a vacuum or an atmosphere ofone or more of the inert gasses, such as argon, nitrogen, helium etc. Inorder to increase the strength of the ceramic insulation, afterpyrolysis, the ceramic insulation or tile can be further fired attemperatures up to about 1500° C. for about 10-60 min in air. To furtherincrease the density and strength of the final ceramic insulationanother embodiment of this invention includes repeating the aboveprocess for up to three times. This multiple processing technique givesthe advantage of tailoring the insulation to specific anticipatedheat-range challenges, and impact strength. For instance, a very-highimpact resistant, but relatively-high density insulation tile, can beprovided for the nose cone area of the vehicle, but tiles of lowerimpact resistance and a much lower density can be provided for the rearand sides of the re-entry vehicle. This procedure allows for a lessexpensive production and minimal weight loading. For more commercialimplementations, such as firewalls, nuclear power facility safety walls,vulcanological applications, and other high temperature applications,similar tailoring can be achieved using this procedure.

In accordance with this invention, the insulation is obtained by thereaction of the tetraalkyoxy silanes with the alkyldialkyoxy silanes inthe presence of an acid or base catalyst in a liquid medium such asalcohol and water to form the siloxane gel which is subsequentlypyrolyzed. The reaction of the silanes is illustrated by the followingformula:

Chemical reaction of tetraalkoxy silanes with dialkoxy silanes:

Ceramic Insulation

The wet gel is a clear siloxane gel containing atoms of silicon, carbon,hydrogen and oxygen. After drying at about 100° C. and pyrolyzing attemperatures ranging up to about 1500° C., in an inert environment, thegel becomes a black ceramic insulation comprising silicon, oxygen andcarbon. This product was found to be stable in an oxidizing environmentat temperatures as high as 1700° C. It was found that the reaction ofthe alkoxy silanes which formed the siloxane gel, dried at ambientpressure with limited shrinkage, and when pyrolyzed in an inertenvironment, resulted in black insulation having improvedhigh-temperature characteristics, light-in-weight, with high-tensilestrength and could be formed into the desired shape for various enduses.

The following Examples illustrate the preparation of the ceramicinsulation materials e.g. ceramic insulation tiles in accordance withthe process of this invention.

EXAMPLE 1.

Tetramethoxysilane (2.0 g) and dimethyldirnethoxysilane (1.0 g) weremixed with 4.0 g of methanol and stirred. After adding 0.5 g. 1.0 N ofammonium hydroxide, the siloxane gel formed within 60 min. One piece ofthe air-dry gel was pyrolyzed at 1200° C. in argon for one hour. Thisblack ceramic insulation was stable at 1600° C. in air.

EXAMPLE 2.

Tetraethoxysilane (2.0 g) and dimethyldiethoxysilane (1.0 g) were mixedwith 6.0 g of ethanol and stirred. After adding 0.5 g of 0.5 N of sodiumhydroxide, the siloxane gel formed within two hours. The air-day gel waspyrolyzed at 1000° C. for one hour. This black ceramic insulation tilewas stable at 1700° C. in air.

EXAMPLE 3.

Tetraethoxysilane (2.0 g) and dimethyldimethoxysilane (0.8 g) were mixedtogether with 5.0 g of ethanol. After several hours, 0.5 g of 0.5 N ofammonium hydroxide was added to the reaction and a gel formed within twohours. The air-dry gel was heated at 1300° C. in argon. The resultantblack ceramic insulation was stable at 1700° C. in air.

EXAMPLE 4.

Tetramethoxysilane (2.0 g) and dimethydiethoxysilane (1.5 g) were mixedtogether with 6.0 g of ethanol and stirred for several hours. Afteradding 0.5 g of 0.5 N sodium hydroxide, a siloxane gel was formed.Pyrolyzing the air-dry gel at 1100° C. in argon resulted in a blackceramic insulation tile which was stable at 1700° C. in air.

EXAMPLE 5.

Tetramethoxysilane (2.0 g), methyldimethoxysilane (0.5 g) andmethyltrimethoxysilane (1.0 g) were mixed with 6.0 g of methanol andstirred at room temperature. After adding 0.5 g of 1.0 N of ammoniumhydroxide, a siloxane gel was formed within 60 min. One piece of theair-dry gel was pyrolyzed at 1200° C. in argon for one hour. This blackceramic insulation was stable at 1600° C. in air.

EXAMPLE 6.

Tetraethoxysilane (2.0 g), dimethyldiethoxysilane (1.0 g) andtrimethylethoxysilane (0.1 g) were mixed with 5.0 g of ethanol and 0.1 gof 0.1 N nitric acid under stirring for 1-20 hours. After adding 1.0 gof 1.0 N of ammonium hydroxide to the reaction mixture, a siloxane gelwas formed within 60 min. Once piece of the air-dry gel was pyrolyzed at1100° C. in argon for one hours. This black ceramic was stable at 1600°C. in air.

It is understood that various other embodiments and modifications in thepractice of the invention will be apparent and can be made by thoseskilled in the art without departing from the scope and spirit of theinvention as set forth in the appended claims.

The invention claimed:
 1. A process of preparing an oxidation-stable,high temperature, porous ceramic insulation which comprises reacting ina liquid medium effective amounts of (a) at least one tetraalkoxy silanehaving the formula: Si(OR¹)₄ wherein R¹ is a saturated or unsaturatedorganic radical having 1 to 12 carbons and (b) at least one dialkoxysilane having the formula: (R²O)₂—Si—R⁴R³ wherein R² is a saturated orunsaturated organic radical having 1 to 12 carbons and R⁴ and R³ aresaturated or unsaturated, either the same or different organic radicalshaving 1 to 12 carbons to obtain a siloxane gel drying said gel and (c)subsequently pyrolyzing the siloxane gel in an inert atmosphere attemperatures ranging from about 900° C. to 1500° C. to produce theporous ceramic insulation.
 2. The process of claim 1 wherein thereaction ratio of the tetraalkoxy silane to the dialkoxy silane is about1.0 part by weight of the tetraalkoxy silane to about 0.1 to 2.0 partsby weight of the dialkoxy silane.
 3. The process of claim 2 wherein thereaction ratio of the tetraalkoxy silane to the dialkoxy silane is about1.0 part by weight of the tetraalkoxy silane to about 0.7 to 1.5 partsby weight of the dialkoxy silane, and the siloxane gel is dried attemperatures ranging from about 100° C. to 150° C.
 4. The process ofclaim 2 wherein R¹ and R² are alkyl radicals of 1 to 8 carbons and R⁴and R³ are alkyl radicals of 1 to 8 carbons.
 5. The process of claim 2wherein R¹, R², R³, and R⁴ are alkyl radicals of 1 to 4 carbons.
 6. Theprocess of claim 2 wherein the liquid medium comprises alcohol.
 7. Theprocess of claim 2 wherein the liquid medium comprises water.
 8. Theprocess of claim 2 wherein the liquid medium comprises a mixture ofalcohol and water wherein the ratio of water to alcohol is about 1.0part by weight of water to 1-100 parts by weight of alcohol.
 9. Theprocess of claim 6 wherein the alcohol comprises at least one aliphaticalcohol having 1 to 8 carbons.
 10. The process of claim 8 wherein themixture of alcohol and water contains a catalytic amount of at least onecatalyst.
 11. The process of claim 10 wherein the catalyst is an acidiccatalyst.
 12. The process of claim 10 wherein the catalyst is a basiccatalyst.
 13. The ceramic insulation obtained by the process of claim 1.14. The ceramic insulation obtained by the process of claim
 2. 15. Theceramic insulation obtained by the process of claim
 8. 16. The processof claim 1 wherein the reaction between the tetraalkoxy silane and thedialkoxy silane contains effective amounts of at least one alkyltrialkoxy silane having the formula: R⁵—Si(OR⁶)₃ wherein R⁵ is asaturated or unsaturated alkyl radical of 1 to 12 carbons and R⁶ is analkyl radical of 1 to 12 carbons.
 17. The process of claim 1 wherein theliquid reaction between the tetraalkoxy silane and the dialkoxy silanecontains at least one alkyl monoalkoxy silane having the formula: R⁷₃—Si—OR⁸ wherein R⁷ is a saturated or unsaturated alkyl radical of 1 to12 carbons and R⁸ is an alkyl radical of 1 to 12 carbons.
 18. Theprocess of claim 16 wherein R⁵ and R⁶ are alkyl radicals of 1-4 carbonsand the ratios in the reaction mixture are about 1.0 part by weight ofthe tetraalkoxy silane to about 0.1 to 2.0 part of weight of thedialkoxy silane to about 0.01 to 0.5 part by weight of the trialkoxysilane.
 19. The process of claim 17 wherein R⁷ and R⁸ are alkyl radicalsof 1-4 carbons and the ratios in the reaction mixture are about 1.0 partby weight of the tetraalkoxy silane to about 0.1 to 2.0 part by weightof the dialkoxy silane to about 0.01 to 0.5 part by weight of the alkylmonoalkoxy silane.